1. Field of the Invention
The present invention relates to a surveying instrument having a sighting telescope and a phase-difference detection type focus detection device for detecting a focus state of the sighting telescope.
2. Description of the Related Art
A conventional surveying instrument such as a total station has a function to measure the distance between two points and also horizontal and vertical angles. Such a surveying instrument is used in various locations, e.g., deep in the mountains, so that the surveying instrument is required to be small and easy to carry. The range of distance measurement of the surveying instrument generally ranges from approximately one meter to several kilometers. To make long-distance measurements possible, a sighting telescope of the surveying instrument is required to be a long-focus and high-resolution sighting telescope having a high magnification. Due to this reason, to make long-distance measurements possible without increasing the dimensions and the weight of the sighting telescope, a long focal length has to be achieved on a short-length sighting telescope. Furthermore, to miniaturize the sighting telescope, the effective aperture of the objective lens of the sighting telescope cannot be made large, which generally increases the f-number to therefore decrease the brightness of the image viewed through the sighting telescope.
In recent years a surveying instrument serving as a distance measuring device which does not require a corner cube reflector to be placed at a point of measurement has been developed. In the optical system of such a surveying instrument, since a measuring light is projected toward a target (sighting object) along the optical axis of the sighting telescope, a light transmitting mirror is disposed in the sighting telescope on the optical axis thereof. The light transmitting mirror is generally formed on the front surface of a transparent plane-parallel plate. A light receiving mirror is formed on the rear surface of the plane-parallel plate to receive the measuring light reflected by the target. Although it is preferable that the light receiving mirror be large to receive a sufficient amount of the measuring light, the optical path extending from the objective lens to the eyepiece of the sighting telescope is interrupted largely by the light receiving mirror if it is large.
Advancements have been made in the development of such a surveying instrument so that it incorporates an autofocus system for automatically focusing the sighting telescope on the target. A phase-difference detection type autofocus system is widely used in the autofocus therefor. In a phase-difference detection type autofocus system, an in-focus state is detected based on the correlation between two images formed by two light bundles which are respectively passed through two different pupil areas upon passing through different portions of an objective lens of the sighting telescope to bring the sighting telescope into focus in accordance with the detected in-focus state. More specifically, the phase-difference detection type autofocus system includes a condenser lens, a pair of separator lenses (a pair of image forming lenses) and a pair of line sensors. The pair of separator lenses are arranged apart from each other by the base length. The image of the target formed on a reference focal plane is separated into two images via the condenser lens and the pair of separator lenses to be respectively formed on the pair of line sensors. A focus state of the sighting telescope is detected based on the correlation between two images respectively formed on the pair of line sensors.
However, if a phase-difference detection type autofocus system is simply incorporated in a sighting telescope with a large f-number which includes a long-focus optical system accommodated in a short-length lens barrel, the following problems occur. Firstly, although an exit pupil is formed at a point which is extremely close to an in-focus point in a long-focus optical system whose mechanical length is short, the aforementioned two different pupil areas cannot be made large since the aforementioned light receiving mirror lies on the optical axis of the objective lens which has a relative large f-number. Furthermore, to miniaturize the surveying instrument, the focal length of each of the aforementioned pair of separator lenses should not be increased because the autofocus system is preferably small in order to miniaturize the surveying instrument. Moreover, since the size of each photoelectric converting element provided on each of the pair of line sensors is invariant, the power of the aforementioned condenser lens of the phase-difference detection type autofocus system has to be greatly increased to ensure that the two light bundles which are passed through the two different pupil areas of the objective lens of the sighting telescope are incident accurately on the effective areas of the pair of line sensors, respectively. However, if the power of the condenser lens is simply increased, aberration increases on each line sensor, and as a result, the performance of autofocus deteriorates. Furthermore, in the case of equipping each of various types of surveying instruments designed for different particular uses with a phase-difference detection type autofocus system, the autofocus system has to be specially designed for each surveying instrument to be suitable therefor so as to satisfy the above described several conditions. This generally requires a long period of time for development of the autofocus systems and a high cost of production.
The present invention has been devised in view of the problems noted above, and accordingly, an object of the present invention is to provide a surveying instrument having a sighting telescope and a phase-difference detection type focus detection device for detecting a focus state of the sighting telescope, wherein the two light bundles which are passed through the two different pupil areas on the objective lens of the sighting telescope are incident accurately on the effective areas of the pair of line sensors, respectively, without increasing aberration on each line sensor, and maintaining a low cost of production without any deterioration in the performance of the autofocus system.
To achieve the objects mentioned above, according to an aspect of the present invention, a surveying instrument is provided, having a sighting optical system and a phase-difference detection type focus detection device which detects a focus state of the sighting optical system from a correlation between a pair of images respectively formed by two light bundles which are respectively passed through two different pupil areas on an objective lens of the sighting optical system, the phase-difference detection type focus detection device including a pair of line sensors, a condenser lens group for condensing the two light bundles which are respectively passed through the two different pupil areas, and a pair of separator lens groups for forming two images of the two light bundles which are passed through the condenser lens group on the pair of line sensors, respectively. The condenser lens group includes more than one sub lens group.
In an embodiment, the phase-difference detection type focus detection device detects the focus state of the sighting optical system on a reference focal plane which is located at a position optically equivalent to a position of a focal plane of the objective lens; and at least one sub lens group of the condenser lens group is disposed on the objective lens side of the reference focal plane.
Preferably, another sub lens group, of the condenser lens group, is provided on the other side of the reference focal plane on which the pair of separator lens groups are disposed. The pair of separator lens groups and the pair of line sensors are provided as elements of a sensor unit which can be adopted for different types of surveying instruments equipped with an autofocus system, and wherein the specification of the at least one sub lens group of the condenser lens group, which is disposed on the side of the reference focal plane on which the objective lens is disposed, varies in accordance with a specification of a surveying instrument for which the sensor unit is adopted.
In an embodiment, the surveying instrument further includes a beam-splitting optical system positioned between the objective lens and an eyepiece of the sighting optical system; wherein the at least one sub lens group of the condenser lens group, which is disposed on the objective lens side of the reference focal plane, is formed on an exit surface of the beam splitting optical system.
In an embodiment, the surveying instrument further includes an optical distance meter which has a light-transmitting optical system for transmitting a measuring light via the objective lens, and a light-receiving optical system for receiving a portion of the measuring light which is reflected by a sighting object and passed through the objective lens.
Preferably, the light-transmitting optical system includes a reflection member provided on an optical axis of the objective lens behind the objective lens.
Preferably, elements of the optical distance meter are arranged in a direction defined along a line which extends across the diameter of the objective lens and intersects the optical axis of the objective lens, and each of the two different pupil areas is determined so as to have an elongated shape extending parallel to this direction.
In an embodiment, the surveying instrument further includes a pair of separator masks which are respectively positioned in the vicinity of the pair of separator lens groups between the pair of separator lens groups and the condenser lens group.
In an embodiment, the surveying instrument includes an erecting optical system. The beam-splitting optical system includes a beam splitting prism which is adhered to a reflection surface of the erecting optical system, a surface of the beam splitting prism which is adhered to the reflection surface being formed as a semitransparent surface which serves as a beam splitting surface. The at least one sub lens group of the condenser lens group, which is disposed on the objective lens side of the reference focal plane, is formed on an exit surface of the beam splitting prism.
In an embodiment, the reference focal plane is formed between the erecting optical system and another sub lens group, of the condenser lens group, which is provided on the other side of the reference focal plane on which the pair of separator lens groups are disposed; wherein the reference focal plane being formed at a position optically equivalent to a position at which a reticle of the sighting optical system is placed.
Preferably, the reflection member is made of a parallel-plate mirror having front and rear surfaces parallel to each other, the reflection member being inclined with respect to the optical axis.
According to another aspect of the present invention a surveying instrument is provided, having a sighting optical system and a phase-difference detection type focus detection device which detects a focus state of the sighting optical system from a correlation between a pair of images respectively formed by two light bundles which are respectively passed through two different pupil areas on an objective lens of the sighting optical system, the phase-difference detection type focus detection device including a pair of line sensors; a pair of separator lenses positioned in front of the pair of line sensors, respectively; a first sub lens group positioned in front of the pair of separator lenses; and a second sub lens group positioned in front of the first sub lens group. The phase-difference detection type focus detection device detects the focus state on a reference focal plane formed between the first sub lens group and the second sub lens group, the reference focal plane being located at a position optically equivalent to a position of a focal plane of the objective lens.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-159764 (filed on May 30, 2000) which is expressly incorporated herein by reference in its entirety.